The Wheel Organ and Hatschek's Groove in the Lancelet, Branchiostoma lanceolatum (Cephalochordata)

Abstract

AbstractThe wheel organ is a specialized epithelium in the roof and sides of the adult lancelet oral cavity. It borders the oral epithelium proper, separated by a thin strip of margin cells which are not ciliated but contain a few large dense‐cored vesicles apically. The wheel organ cells are tall and strongly ciliated and have dark, heterochromatin‐rich nuclei. Dorsally, and slightly paramedially, the organ is further specialized, forming the so‐called Hatschek's groove (pit), which consists of two ciliated cell types. The first type synthesizes a dense granular material, the granules being approximately 95 nm in diameter. This is stored basally and apparently it is also released through the basal cell membrane into the blood cavities. The cells at the bottom of Hatschek's groove have peculiar rod‐shaped apical cellular regions. Each cell bears one tall cilium surrounded by microvilli and it is apparently involved in the production of secretory material into the groove. It is evident that the histology, and probably also the function, of the wheel organ and its groove is much more complex than hitherto believed.